The toxicity of hydrogen sulfide in hydrocarbons or hydrocarbon streams is well known in the industry and considerable expense and efforts are expended annually to reduce its content to a safe level. Many regulations require pipeline gas to contain no more than 4 ppm hydrogen sulfide.
In large production facilities, it is generally more economical to install a regenerative system for treating hydrogen sulphide streams. These systems typically employ a compound used in an absorption tower to contact the produced fluids and selectively absorb the hydrogen sulfide and possibly other toxic materials such as carbon dioxide and mercaptans. The absorption compound is then regenerated and reused in the system. Typical hydrogen sulfide absorption materials include alkanolamines, hindered amines, and the like, i.e. nitrogen containing compounds. However, such approach is not economically feasible for development stage of a field or in small producing fields.
For development stage of a field or in small producing fields where regenerative systems are not economical, it is necessary to treat the sour hydrocarbon production with non-regenerative scavengers.
The U.S. Pat. No. 1,991,765 [US'765] disclosed use of reaction of aldehyde and hydrosulfide in aqueous solution having pH between 2 to 12. Thereafter, use of aldehydes to remove or scavenge hydrogen sulfide was reported in many patents. Mainly aldehydes including formaldehyde, or glyoxal, or formaldehyde in combination with other aldehydes, or glyoxal in combination with other aldehydes have been used as hydrogen sulfide scavengers/removing agents. In the formaldehyde type reaction, the reaction produces a chemical complex known as formthionals (e.g., trithiane).
The non-regenerative scavengers for small plant hydrogen sulfide removal fall into four groups: aldehyde based, metallic oxide based, caustic based, and other processes. In the removal of hydrogen sulfide by non-regenerative scavengers, the scavenger reacts with the hydrogen sulfide to form a nontoxic compound or a compound which can be removed from the hydrocarbon.
The U.S. Pat. No. 4,680,127 [US'127] reported use of glyoxal or glyoxal in combination with other aldehydes in small amounts, which resulted in scavenging of hydrogen sulfide by forming water soluble products. However, the main problem of this method is that the resulted water soluble products were stable only in alkaline pH of about 9, and decomposed in acidic pH of about 4.5 to 5.5.
The solution to above problem of US'127 was provided by U.S. Pat. No. 5,085,842 [US'842] which reported use of glyoxal, but in very high amounts at least of 15% by weight, preferably of 25 to 45% by weight to form water insoluble products. The main problem of this solution is that glyoxal has to be employed in very high amounts, which also makes the process highly uneconomical. Additional problem of this method is that it results in water insoluble products, which are prone to get deposited in the vessels and cause fouling meaning thereby additional anti-fouling additive will be required. Accordingly, as per inventor of present invention, this method is neither economical nor industrially feasible and convenient.
The U.S. Pat. No. 6,666,975 [US'975] also reported use of glyoxal, but with aim to provide a method to reduce emission of hydrogen sulfide odor wherein products formed are water soluble and non-volatile. The US'975 does not aim to overcome problem of fouling in treatment of hydrocarbons which may be caused due to water insoluble products formed by use of glyoxal in higher amounts as reported in US'842, but only aims to avoid handling problems of glyoxal without any disclosure or teaching that how one can achieve hydrogen sulfide scavenging without facing a) problem of fouling which may be caused by employing method of US'842 and b) problem of decomposition of products which may be water soluble products but decompose in acidic pH which may be caused by employing method of US'127. Even the US'975 does not discuss US'842 and US'127.